A Practical Vision for OTN

 A Pr act ctii cal Vi Viss i on for for Opti cal Tr ansp spo or t N etwor ki ng Paul Pa ul Bo Bonenfa nenfant, nt, Cur Cur t New N ewto ton, n, Kev K evii n Sp S par k s , Ev E ve Varr ma Va ma,, and and Ro Rod Alfer Alfer ness

WHI T E P AP E R ET WORKI N G GROUP OPTI CAL N ETW

ABSTRACT

I NTRODUCTION

This white paper presents a practical vision for  Optic Op tical al Tr Trans ansp por t N etwo tworr k s ( OTN OTNs). s). I n this vision, OTNs will evolve from point-to-point  DWDM r emedi die es to sc scalable, alable, ro r obust opti opticcalnetwo ne tworr k i ng ap appli pliccati atio ons that cate cater to a wide wi de variety of client signals having equally-varied   se  s ervic rvice e re req quir uire ement ntss. This This paper pla placces the evolution of OTNs in context with other  netwo ne tworr ki ng tr tre ends, and addr addre ess sse es the the fifive ve  facctor s most cent  fa ntral ral to re rea al-wo l-world rld Optic ica al Transport Networking – span engineering, maintenance, survivability, interoperability, and se ser vi vicce tr transp anspar are enc ncy. y. N OTE : A conde ndense nsed  d  version of this white paper appeared as the introduction to the Bell Labs Technical Journal  Spe  Sp ecial I ssue on Optic ica al N etwo worr ki king, ng, Jan Jan.-Fe .-Feb b. 1999.

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ommercial lightwave communications over fiber-optic cable began some twenty years ago. Since then, researchers have held a vision of  accessing a larger fraction of the theoretical 50 THz of avail availab able le infor i nforma mation tion bandwi bandwidth dth that th at singlesinglemode fiber offers. In more recent years, accelerated resea research rch has explo explored red sophisticated sophisticated photoni photonic c devices and techniques that would allow the transport and routing of signals in the optical domain. At the same time, network operators, spurred by persistent projections of vastly increased demands for bandwidth for services of widely varying characteristics, have been driven by a vision of a network that could gracefully evolve to meet unknown future demands for increased flexibility, capacity, and reliability. Such networks, in addition addition to providing provi ding hig hi gh-s h- speed peed transmiss transmission capabilities, would need to incorporate multiplexing and routing techniques appropriate for ultra-broadband services. It was recognized that the efficient implementation of such transmission/switching functions could dramatically reduce overall network cost.

As evidenced by the Special Issue of the B ell Labs Technical Journal on Packet Networking [1], the persistent projections of earlier years have finally become a reality. The current unprecedented demand for network capacity is mostly driven by the rapidly growing demand for packet-based services – in particular, by Internet/Intranet-based applications. A conservative estimate of Internet traffic growth is that it doubles every six months. If  this growth rate is accurate, and continues, the aggregate bandwidth required for the Internet by the year 2005 in the US will be in excess of 280 Tbps [2]. Transport networks currently optimized for a mix of narrowband and wideband servi service ces s (64 kbps kbps to 2 M bps) bps) must become become optimized for much larger channels carrying broadband data, voice, and video. The combination of an unprecedented demand for new capacity and the emergence of very-high bandwidth applications, at a time when many network service providers are experiencing high utilization rates of  their existing fibers, has led network planners to look for the most expedient and cost-effective means of increasing capacity. Hence, the research vision of virtually infinite information bandwidth and transport in the optical domain domain,, together together with wi th network operator needs needs for dramatically increased network capacity and an

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LUCENT TECHNOLOGIES OPTICAL NETWORKING

associated broadband infrastructure, have become mutually reinforcing thrusts. With individual channels in the transport network growing to gigabits/second, and with optical technology becoming increasingly viable and cost-effective, we are now witnessing rapid large-scale deployment of  Waveleng Wavelength th Divis Divi sion M ultiplexe ul tiplexed d (WDM (W DM)) syste systems ms worldwide. worl dwide. Network operators operators are are relying relyin g on WDM WDM extensively extensively to expand expand trans tr ansmiss mission ion capac capacity ity on long-haul and fiber-limited routes, currently achieving throughput in excess of 400 Gbps per fiber. Because the primary short-term need being addressed has been capacity gain, these deployed systems have been point-to-point line systems. Beyond thes these poin point-tot-to- point WDM W DM applications, applications, the next evolutionary step in reliable, scalable transport networks will be optical networking. But what exactly is optical networking? Like L ike many many other networking techniques, optical networking implements implements functionality fun ctionality for transmiss transmission ion,, multiplexing, routing, supervision, and survivability for a wide range of client signals. As described in a previous issue of the Bell Labs Tec Techni cal J ournal ur nal [3], optical networking operates predominantly in the optical domain, where the unit of bandwidth granularity – the optical channel – is i s much larger larger than than in Time Division M ultiplexe ulti plexed d (TDM ) networks network s. In I n an optica opticall network, network , the optica opticall channel (“frequency slot”) may be considered as analogous to a time slot slot in a TDM system, ystem, with wi th optical-network elements manipulating optical channels (“frequency slots”) similar to the way  TDM ele elem ments manipulate nipulate time time slots. In an idealized optical network, there are no analog engineering constraints on optical-channel  service rvicerouting; routi ng; the network is complete completely ly se transparent , that is, free from service-specific functions that can limit flexibility. In addition, ubiquitous ubiqui tous waveleng wavelength th conversion conversion (interc (i nterchang hange) e) is is available to minimize stranded capacity, and there is support for a multi-vendor environment. Of  course, the reality is more challenging. Public telecommunications relies on an extremely diverse network of networks, with widely varying topologies, deployed technologies, services, and underlying business models. A practical vision for optical networking is one that goes beyond pointto-poin to- pointt WDM WDM transmiss transmission ion to address address the practical practical

needs of this diverse networking environment. It encompasses the progression of applications from long-haul capacity expansions toward a unified end-to-end optical network, spanning access, metro, and long-haul domains. Such a vision also describes how optical networking supports an expanding variety of client signals that have equally varied service requirements – flexibility, scalability, and survivability, coupled with bit-rate and protocol independence.  This pape paperr foc focuse uses specifica ifically on on optic optica alnetworking networki ng soluti solutions ons for transport transport appli applica cations tions1, or or the Optical Transport Network  (OTN). (OTN) . Optical Optical techniques have been evolving for a wide range of  networking networki ng applica applications. tions. Howe How ever, owing owi ng to the unique combination of technology and business tr anspo or t netwo networr k s specifically that forces, it is on transp optical networking offers the most immediate and extensive payoffs. While much of this paper is relevant to the full range of optical-networking applications, including enterprise and residential access, it specifically addresses transport applications. In summary, optical transport networking has come of age. A practical vision for optical transport networks – one that is widely deployable, highly reliable and maintainable, readily evolvable, and demonstrably cost – effective - is now at hand. This paper lays out this practical vision, placing it in context with other networking trends. In addition, this paper addresses the most critical factors to consider in architecting optical transport networks.

OPTICAL TRANSPORT NETWORKING

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t is essential to clearly define what is meant by the term Optical Transport Networking. After all, the ability to support optical transmission is not new – SONET/SDH equipment has been used successfully in the construction of single-channel optical transmission systems for some time. However, SONET/SDH networks and optical networks differ in several respects; in particular, in how they support capacity expansion and channel routing. In SONET/SDH networks, once the transmission rate of a network’s single optical channel has been maximized, capacity expansion

1 Transp Tr anspo or t netwo networr ks pro pr ovide vi de the unde under lying lyi ng highhi gh-ccapac apaci ty infr i nfras astru truccture tur e for for cor e interoffic interoffi ce, metro tr opolitan li tan inte i nterr offic ffi ce, and  broa broadba dband nd business-acc business-access networks. twork s. We We disti distingu nguii sh these these transpo tr anspor t-ne t-n etworki twork i ng applic appli cations ations from fr om other appli appli cations, ations, such such as re r eside sidential nti al access and enter pri se networki twork i ng, because each ach may may have quite qui te di di ffere ffer ent ar chite hi teccture tur e re requir qui r ements fo for  capac apaci ty, traffi tr affi c managem management, physical topologie logi es, oper ations, ations, and re r eliabil li abilii ty. A t the same time, time, there may may also besome some technologi es and other aspects that are ar e common mon across across all the th ese appli applications. cations.

LUCENT TECHNOLOGIES OPTICAL NETWORKING

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OTN Client Connection (Optical Channel) Optical Subnetworks

Optical Subnetworks

Optical Subnetworks

OTN Client OTN Client (e.g. SONET/SDH, IP, ATM)

Metro

Core

M et r o

Optical Transport Network (OTN) Optical layer Cross Connect (OLXC)

Opti Op tica call Add Add/D /Dro rop p Mul Multi tipl plex exer er (O (OAD ADM) M)

Fig ure 1. Optical Optical Transpo Transport rt Network wit h Opti cal Channe Channell Routing involves adding new transmission systems in parallel over separate fibers. In optical networks, capacity expansion involves simply adding wavelengths within the same fiber (up to some predefined maximum) and transmission system.  The routing routing functio functions ns for SONET/S SONET/SDH DH ne networks tworks are performed by means of time slots, whereas the routing functions for optical networks are performed by means of optical channels (frequency slots) between wavelengths of various frequencies.  The use use of WDM – more more spec pecifica ifically, the rapid rapid deployme deployment nt of point-to-poin point- to-pointt WDM line li ne sys syste tems ms in telecommunications networks worldwide – is viewed as the first step toward optical transport networking. networki ng. Whil Wh ile e WDM line li ne syste ystems ms alone support little in terms of networking functionality, the eleme elements nts for WDM W DM optical transport transport networking are on the horizon. WDM line systems with wi th fixed f ixed wavelengt wavelength h add/ add/drop drop capa capabili bility ty are being deployed, and optical-network elements with nodal featur features es,, such as optical optical add/ add/ drop multiplexe multi plexers rs (OA DMs DM s) and optical optical cross cross-connects (OXCs) – employing either electrical or optical switching matrices – have been reported to be in laboratory and field trials. The ability of these WDM WDM nodal elements elements to add, add, drop, and in effect effect construct construct optical channelchannel-routed routed networks allows allow s for the manipulation of optical channels in WDM networks, just as time slots are manipulated in  TDM  TDM netwo networks rks toda today y (se (see Figure Figure 1). Optical transport networking is defined as the abil ability ity to construct construct WDM networks having having advanced features, such as optical-channel routing and switching, that support flexible, scalable, and reliable transport of a wide variety of client signals at unprecedented bandwidth

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granularities – upwards of tens of Gbps per optical channel.

NEX EXT T GENERATION NETWORKS

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his section introduces how our practical vision of optical transport networking relates to Lucent’s vision of the next generation network, as presented in the Bell Labs Technical Journal Special Issue on Packet Networking [1]. We first summarize the role of optical transport networking in the next generation network, and then examine the evolution of transport networks from single-w ingle-wav ave elength length TDM networks networks to WDM line lin e systems, and ultimately to the vision of optical transport networking at the optical-channel level.

OPTICAL  TRANSPORT NET ETWO WORKIN RKING G IN GENERATION NETWORK 

THE

NEXT

 Tod  Today’s y’s TDM TDM -bas -based tra trans nsp port netwo networks rks have have been designed to provide an assured level of  performance and reliability for predominantly voice and leased-line services. Proven technologies, such as SONET/SDH, have been widely deployed in the current transport infrastructure, providing high-capacity transport, scalable to gigabit-persecond rates, with excellent jitter, wander, and error performance for 64 kbps voice connections and leased-line applications. SONET/SDH selfhealing rings enable service-level recovery within tens of milliseconds following network failures. All of these features are supported by well-established global standards, enabling a high degree of multivendor interoperability. To summarize, SONET/SDH has been the transport-networking technology of choice for a world dominated by circuit-voice and leased-line services.

LUCENT TECHNOLOGIES OPTICAL NETWORKING

I n contrast with wi th today’s TDM - base based d transport transport networks – and, to some extent, with ATM networks – legacy “best-effort” IP networks generally lack the means to guarantee high reliab reli abil ility ity and predictable predictable performance. performance. The best-effort service provided by most legacy data networks, with unpredictable delay, jitter, and packet loss, is the price paid to achieve maximum link utilization through statistical multiplexing. Link utilization (for example, the number of users per unit of bandwidth) has been an important figure of merit for data networks, because the links are usually carried on leased circuits through the  TDM trans transpo port rt netwo network. rk. While toda today’ y’s s data data networks provide excellent connectivity, they do not enable controllable distribution of network resources among traffic from competing users. Given the inherently bursty statistics of packet data traffic, the fixed fix ed bandwidth bandwidth pipes of TDM transport transport may not be an ideally efficient solution. However, this inefficiency has traditionally been considered of less importance than the network reliability and congestioncongestion- isolation isolation fea featur tures es that a TDM - base based d transport network provides.  The surging urging dema demand nd for high-ba high-band ndwidt width h and and differentiated data services is now challenging this dual-architecture dual- architecture model model of TDM - bas based transport and best-effort packet data networks. It is not cost-effective to extend the usefulness of best-effort networking by over-provisioning network bandwidth and keeping the network lightly loaded. Further, this approach cannot always be achieved or guaranteed due to spotty demand growth, and is a particular issue for the network access domain that is most sensitive to economic constraints of  under-used facilities. For a commercial network, an additional drawback of the best-effort model is that the network relies only on user (customer) cooperation for congestion control, by assuming that end users will slow down their transmission rates when significant congestion is detected. As a result, in general, data-service providers today do not have the network infrastructure support to provide customer-specific, differentiated-service guarantees and corresponding service-level agreements. With regard to satisfying these new network abs Tec Techni cal Jo J ournal ur nal Special requirements, the B ell L abs Issue on Packet Networking [1] described a new data-centric optical transport network architecture.  This next next ge genera neratio tion n netwo network rk will dra drama matic tica ally increase, and maximally share, backbone network-infrastructure capacity, and provide

sophisticated service differentiation for emerging data applications. Complementing the many service-layer enhancements, optical transport networking will provide a unified, optimized layer of high-capacity, high-reliability bandwidth management, and create “Optical Data Networking” solutions for higher-capacity data servi service ces s, with wi th guaranteed uaranteed quality. quali ty.

 THE VAL ALUE UE OF TRANSPORT NETWO WOR RKIN KING G IN A DAT ATA A -CENTRIC WORLD Network architectures for cost-effective, reliable, and scalable evolution employ both transport networking and enhanced service layers, working together in a complementary and interoperable fashion. Transport networking, whether based on SONET/ SDH or optical optical technol technolog ogy, y, enables enables the service layers to operate more effectively, freeing them from constraints of physical topology to focus on the sufficiently large challenge of meeting service requirements. This point is fundamental to an understanding of our vision of optical networking.  Trans  Transpo port rt netwo networking rking offe offers four four prim prima ary benefits nefits:: • It is the only solution solution for fast fast survivability, survivability, and it supports service-layer restoration with efficient shared-protection architectures. • I t enables enables servic service-laye e-layerr scalab calabil ility, ity, controlling controlli ng the pace of service-layer upgrades by freeing the service-layer logical topology from the network’s physica physicall-li link nk topology. topology. • I t enables enables you to achieve achieves s a lower-cost lower-cost network, network , especially when upgrade-fueled lifecycle costs are included. • I t es establishe tablishes s a future-rea futur e-ready dy unifying unif ying infrastructure for efficient multi-service networking in an unpredictable service environment.  Thes  These benefit nefits s are appa pparent rent whe when cons conside idering ring the cost and functionality of intermediate nodes along high-capacity, long-haul service routes. As shown in Figure 2, transport networking permits a flexible, cost-effective end-to-end connection without incurring the cost of service-layer processing at the intermediate nodes. As traffic demand grows, the transport network evolves from managing broadband circuits toward managing optical channels. channels.

LUCENT TECHNOLOGIES OPTICAL NETWORKING

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Service Layer Networking (IP, ATM)

larger-capacity channels carrying broadband data, voice, and and video. WDM WD M ’s increasingly increasingly hig hi gh channel counts and large information capacity are an excellent solution to these needs.

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OPTICAL CHANNEL BANDWIDTH M ANAGEMENT

Optical Networking

Optical Networking Element Element (OADM , OXC)

Fig ure 2. Compleme Complementary ntary Opti cal cal Transport  Transport  Network ing and Service Service Lay Layer er Networ ki ng However, what if optical transport never progresses beyond beyond pointpoint- to-point to-poi nt WDM interconnects interconnects between service-layer elements? Point-to-point transport, rather than full-functionality transport networking, places new demands on the service layer for through-traffic networking and survivability, on top of the rapidly growing service layer requirements. Already, growth and churn are forcing many data-networking systems to be replaced or upgraded every 12 to 18 months. Analysis of real networks suggests that without the support of transport networking, the aggregate service-layer capacity more than doubles in order to compensate, and likely shortens useful service-layer lifecycles even further.  Thus,  Thus, eve even n in em emerging rging broadb roadba and data data-networking applications, a full-functionality transport layer provides considerable value. From an optical transport networking perspective, the essential elements are efficient and cost-effective capacity expansion, flexible optical-channel bandwidth management, and survivability mechanisms to support improved reliability of data networks.

 TRANSPORT CAPACITY EXPANSION As discussed earlier, the combination of an unprecedented demand for new capacity and maximal usage of existing cable systems has led network planners to look for the most expedient and cost-effective means to increase capacity. Thus, embedded carriers must evolve their current transport architectures, currently optimized for a mix of narrowband and wideband services (64 kbps kbps to 2 M bps), bps), to become become optimized for

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Bandwidth management functions – routing channels channels throug throu gh a network using add/drop, add/drop, cross-connect, and interchange techniques – are essential for real networks, because they provide an efficient means to accommodate growth and churn. Several levels of bandwidth-management granularity are necessary, reflecting a range of  service requirements and the increasing degree of  traffic aggregation toward the core of the network2. While the explosion in data traffic is driving drivi ng enhanced enhanced requir requirem eme ents for fine-gra fin e-granul nularity arity bandw bandwidth idth managem management ent in the th e I P and ATM service layers, it is simultaneously creating a rapid proliferation of ultra-broadband optical channels. Optical transport networking will provide flexible management for these optical channels, complementing SONET/SDH’s bandwidth management for lower-bandwidth transport channels.

NETWORK  RELIABILITY How will next generation networks deliver data services with the reliability of today’s voice network? While service-layer enhancements will improve packet loss and delay, transport-network survivability provides the best first line of defense against major network faults. Survivable transport networks are essential for two reasons – fastest recovery from faults such as fiber cuts, and efficient capacity utilization through shared-protection mechanisms. For broadband data networks, recovery time becomes even more critical as each fiber span carries an ever-increasing amount of  informa inf ormation. tion. Transport Transport protectionprotection- switch wi tch times, times, on the order of tens to hundreds of milliseconds, are orders of magnitude faster than for typical service-layer scheme, giving multi-service providers important options to meet premium Service Level Agreements. In addition to delivering the fastest recovery from faults, the next generation network will include advances in survivable network efficiency and flexibility. The survivable optical network will be able to optimize the use of   2 For a mor mor e complete trea tr eatme tment of bandwi bandwi dth management, see [4].

LUCENT TECHNOLOGIES OPTICAL NETWORKING

protection bandwidth, as well service routing, on an optical-channel basis.

different ways of passing from ingress to egress. To satisfy varied requirements, there are several ring-based protection schemes, including SONET 2-fiber and 4-fiber Bi-directional Line Switched Ring Rin gs (equi (equiva valent lent to SDH M ultiplex ul tiplex Section Section Shared Shared Protection Rings) and SONET Unidirectional Path Switched Rings (akin to SDH 1+1 Sub-Network Connection Protection in a physical ring). For long-span applications, regenerators can be placed at approximately 40 - 80 km spacing between A DM nodes. nodes. Rings Ri ngs may may also exi exis st in in open configurations (linear add/drop, open ring). Or, in the simpl simples estt form of ring, ri ng, the A DM can can be configured in a point-to-point configuration (for example, example, 1+1 SDH Mul M ulti tiplex plex Sectio Section n Protection/SONET Line Protection).

LEGACY TRANSPORT NETWORK  EVOLUTION WDM is a revolutionary revolution ary technology technology in many respects. However, its impact on transport networking networki ng is largely largely evolutionary, llea eading ding to the th e next phase of capacity expansion and service independence. From a high-level architectural perspective, optical transport networks will follow many of the same architectural principles as their SONET/ SONE T/SDH SDH predece predecess ssors. ors. Therefore, Th erefore, a basic basic understanding understanding of SONET/ SONE T/SDH SDH transport transport networking (Figure 3) is essential to understanding the nature of practical optical transport networks. A decade of SONET/SDH deployment has established several types of network elements and network topologies as the basis for transport networking. There are three broad classes of  network eleme elements: nts: multiplexe multi plexers, rs, including i ncluding  Term  Termina inall Multip M ultiple lexe xers rs (TMs) (TMs) and and Add/Drop Add/Drop M ultipl ultiplexe exers rs (ADM (A DMs); s); rege regenerators nerators;; and and Digital Cross-connect Systems (DCSs). Among numerous network topologies, survivable rings have proven to be particularly useful. A ring ri ng compri comprise ses s a set set of ADM A DM s that allow traffic traffi c to enter and exit the ring, interconnected in a loop configuration. The main advantage of the ring topology is protection; the ring offers traffic two

Digital Cross-connect Systems groom traffic at various rates to ensure that network facilities are well used, and are key elements in mesh-based restoration architectures. In a typical application, lowlow - speed peed signals signals dropped dropped from an ADM A DM are routed through a digital cross-connect system, where they may be groomed (that is, separated and recombined according to service type or destination), and handed off to another ADM.  Tradit  Traditiona ionally, lly, cross ross-conne -connec ct syste ystem ms have have not performed the ring facility termination functions ass associated ociated with wi th ADM A DMs s (for example, example, Multi M ultiplex plex Section Section protection protection switching), wi tching), althoug althou gh Luce Lu cent nt  Tec  Technolog hnologies ies’ next next-g -ge enera neratio tion n tra trans nspo port rt syste ystem m,

PSTN tru trun n s. In Inte tern rnet et ac

one on e, FR/AT /ATM M

ac

one on e

- - - - Core TDM channels : 45 - 622 M bit bit/s /s

ADM

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MS-SPRing 2.4 - 10 Gb/s

ADM

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ADM

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Centrall O ff ice Centra ices s

ADM A DM

Metro Interoffice

Access

MS-SPRing 2.4 - 10 Gb/s

ADM

ADM

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A DM

DCS

ccess ess TDM ...... A cc Channels : 1.5 - 155 Mbi t/ t/ss Centrall O ff ice Centra ices s

ADM

(e.g. PSTN access trunks, tru nks, ISP access access lines)

SNCP ring 155 - 622 Mb/s

ADM

ADM

 SONET/SDH   SONET/SDH  ADM : Add/Drop Multiplexer DCS : Digital Cross-Connect System TM : Terminal Multiplexer

RT sites

Fig ure 3. SONET SONET Transport Transport Networ k A rchitectures rchitectures

LUCENT TECHNOLOGIES OPTICAL NETWORKING

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Fiber exhaust and ultra-broadband channels (>2.4 Gbit/s) drive deployment deployment of pt-pt DWD M  for capacity capacity expansion expansion

Internet  IP VPN  FR/A FR/ A TM

ATM IP

ATM

IP

Pt-Ptt DW DM Pt-P OLS

Pt-Ptt DW DM Pt-P OA

ADM

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DCS

 TM

 TM

DCS

ADM  TM DCS

 SONET/SDH  ADM : Add/Drop Multiplexer : Digital Cross-Connect Syste DCS : Terminal Multiplexer TM DWDM OLS OA

: Optical Line System : Optical Amplifier

Figure 4: Point-to-Point WDM for Core/Long Haul Capacity Expansions the WaveStar™ WaveStar™ BandWidth BandWi dth M anag anager, integ in tegrate rates s this functionality fu nctionality and eli eliminate minates s standa standalon lone e ADM s. A DM s and and reg r egenera enerators tors are most most often often deployed in single-vendor configurations for management and interworking simplification. On the other hand, DCS systems must support more extensive multi-vendor inter-working, because they serve a “hub” role in a central office, terminating electrical and optical signals from the multi-vendor network. It should be noted that the variety of survivability architectures deployed within inter-office and long-haul backbone networks, as well as within access transport networks, reflects the critical need for high networking reliability and performance. Along with the dramatic increase in network capacity that has taken place over the last decade, customers have been concerned with such issues as: surviving catastrophic failures in cable-facility and equipment sites; the restoration times of  critical circuits when a wire center failure occurs; and new survivability levels for applications such as automatic teller machines, local area network interconnection, and mainframe-to-mainframe computer computer links. li nks. From the perspective of evolving to optical networking, many SONET/SDH transport functions and architecture principles will transfer to new optical transport networks as the basic unit of  transport bandwidth shifts from time slot to optical channel (frequency slot).

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 THE NEXT PHASE : POINT- T  TO O- POINT WD WDM M Point- to-point to-poi nt WDM W DM repres represe ents the next phas phase in the evolution of transport networking, and the first realization of optical networks based on multiple wavelengths. wavelengths. WDM deploy deployme ment nt is grow growin ing g rapidly because it is often a more cost-effective way to expand capacity than any alternatives, such as adding fiber or replacing current-capacity (for example example,, 2.5 Gbps Gbps) Time Ti me Divis Divi sion M ultiplexing ul tiplexing (TDM (TDM ) transport syste systems ms with wi th new, highe hi gherr-rate rate  TDM  TDM syste ystem ms. New New fibe fiber is partic rticula ularly rly exp expe ensive nsive for long routes, and its installation can take too long to satisfy some customers. Upgrading TDM systems often turns into wholesale replacement, and TDM techn technolo ology gy is no longer l onger keepin keeping g pac pace e with the emerging ultra-broadband data backbone demands. Compared to new ffiber iber or upgraded upgraded TDM systems systems,, point-topoint- to-point point WDM W DM system ystems s (Figure (F igure 4) 4) offer off er a superior value proposition, in particular for the long-distance environment, where electronic regenera regenerators tors are requir r equired ed for long, long, “repe “repeate atered” red” transmission spans.  This value value prop propos osition ition is based on the the following following:: • High-capacity path routing: the high TDM capacity per optical channel (frequency slot) allows “direct feed” from IP routers and ATM switches with ultra-broadband interfaces. • Electronic regenerator cost savings: for an N-channel system, a single Optical Amplifier (OA) replaces N electronic regenerators at each

LUCENT TECHNOLOGIES OPTICAL NETWORKING

repeater site. • Greater Greater transport transport capac capacit ity y per per fibe fi ber: r: the addition addition of WDM Terminal Terminal Mul M ultiplexers tiplexers (WDM (W DM-TM -TM s) exploits the inhere inh erent nt capa capacity city of  optical fiber, which is vastly underused by single-channel single-chann el optical optical transmission ion systems ystems.. • Preservation Preservation of inves investment tment in exi exis sting ti ng equipment: existin existing g TDM equi equipme pment nt nee n eed d not be replaced, but continues to operate in parallel with wi th other, new TDM equipment equipment on the sam same e fiber. • Unification of transport for all services: different types of services, both existing and new, can use the same fiber, virtually independent of bit rate or protocol. I n short, WDM W DM allows se service providers providers to tap tap into in to the full capacity of their existing fiber plant, thus maximizing the return on existing facilities. In addition, the service provider has the flexibility of  adding new services onto the existing fiber to accommodate new capacity demands. As illustrated in Figure 4, point-to-point WDM can be deployed deployed on individual indi vidual spans spans of a TDM ring rin g topology, as well as on linear networks. In either case case,, the existing TDM network continues contin ues to operate as if it were still the only application on the fiber (for example, TDM protection protection is stil stilll active), and new applications can be added to new wavelengths without impacting the existing TDM traffic. Several fundamental technology advances, particularly dispersion-managed fibers coupled with Erbium Doped Fiber Amplifiers (EDFAs), have helped make make WDM WDM a cost-effective, cost-effective, deployable deployable techn technolo ology. gy. Commerciall Commercially y avail availab able le WDM WDM systems systems carry 16 or more wavelengths, each capable of  carrying up to a 2.5-Gbps or 10-Gbps signal. Furth Further, er, the rapid pace pace of WDM WDM - related related technol technology ogy advances has resulted in the announcement of  higher and higher channel-count systems. For example, Lucent Technologies recently unveiled an 80-channel, global optical networking system, the WaveStar™ OLS 400G, delivering up to 400 Gbps on a single fiber. Other advances, such as Lucent  Tec  Technolog hnologie ies s AllWave AllWave™ fibe fiber, are are ope opening up previously unusable parts of the optical spectrum for WDM W DM transport, transport, permittin permitting g even even more wavelengths on each fiber.

WDM WDM represents represents the first step step toward toward optical networki networ king, ng, becaus because e it employ employs s wavelengthwavelength-ba base sed d transport. However, in these initial point-to-point applications, most of the transport-networking functionality is provided by the underlying TDM systems systems that use the WDM span. span. As A s networ network k traffic traffic grows and and WDM WDM deplo deployment yment conti continu nues es,, optical channels will increasingly become the fundamental medium for exchange in networks. Transportnetworking functions will migrate into the optical layer, and carriers will begin to manage capacity at the optical-channel level. Consequently, the application of WDM in the transport network will quickly evolve from point-to-point capacity expansion to scalable and robust optical transport networking applications that cater to an expanding variety of client signals with equally varied service requirements.

 THE FUTURE : OPTICAL TRANSPORT NETWORK  EVOLUTION As previously indicated, Optical Transport Networking (OTN) represents a natural next step in the evolution of transport networking. As an evolutionary result, optical transport networks will follow many of the same high-level architectural principles as followed by SONET/SDH transport networks. For instance, both SONET/SDH and OTN are connectionconnection-oriented oriented,, multiplexe multi plexed d net n etwork works s.  Thus, opt optic ica al-netwo l-network rk top topolo olog gies ies and surviva urvivab bility schemes will closely resemble – if not mirror – those of SONET/SDH TDM networks. At the same time, there are some important, if  subtle, distinctions between optical and SONET/SDH networks. The major differences derive from the particular form of “multiplexing” technology technology used used:: digital digital Time Ti me Divis Divi sion M ultiplexing ulti plexing for SONET/SDH versus analog Wavelength Division M ultiplexing for OTN. The digital versus analog distinction turns out to have a profound effect on the fundame fundamental ntal cost/pe cost/ perfor rforma mance nce trade-off trade-offs s in many aspects of OTN network and system design. In particular, the complexities associated with analog network engineering and maintenance implications, as outlined in Section 4, account for the majority of the challenges associated with optical transport networking.  To satisfy tisfy the short-te hort-term rm nee need for capac pacity gain, large-scale deployment of WDM point-to-point line systems will continue. As the number of  wavelengths grows, and as the distance between terminals grows, there will be an increasing need

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-OADM

Core/ Long Haul

Core Optical Op tical Channels : 2.4 G bit bit//s and up (next generation packet core ; legacy networks)

Ring 40 - 80 λ

OADM

OXC

OLS OADM

OXC

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OLS

Mesh 40 - 80 λ

OXC

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Central Offices

OADM OA DM

Metro Interoffice

OA DM

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ccess ess ...... A cc    

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Optical Channels 155 Mbi t/ t/ss to Gbit/s

OADM

Access

OADM

OADM

8 - 16 λ

Optical Networking OXC : Optical Cross Connect   OADM : Optical Add/drop M ultiplexe OLS : Opti cal Line System System OA : Optical Amplifier

OADM

OADM

Fig ure 5. O ptical ptical Transp Transport ort N etwork A rchitecture rchitecture to add and/or drop wave wavelengths lengths at at interme i ntermediate diate sites. Hence, flexible, reconfigurable optical add/ add/drop drop mul multiplexers tiplexers (OA DM s) will wi ll become become integ in tegral ral elements elements of WDM WDM networks. networks. As more wavelengths become deployed in carrier networks, there will be an increased need to manage capacity, as well as to hand off signals between networks, at the optical-channel level. In much the same way that digital cross-connects emerged to manage capacity at the electrical layer, optical cross-connects (OXCs) will emerge to manage capacity at the optical layer. Similar to their electrical counterparts, OXCs will be required to support a multi-vendor environment, while individual linear and ring subnetworks based on optical line li ne sys system tem (OLS) (OLS) terminals termin als and OADM OA DMs s will typically be single-vendor. Figure 5 depicts an optical transport network for core, metro-interoffice, and high-capacity business-access applications. Initially, the need for optical-layer bandwidth management will be most acute in the core (long distance) environment. The logical mesh-based connectivity found in the core will be supported by way of physical topologies, includi in cluding ng OADM OA DM-- base based d shared shared protection rings ri ngs,, and OXCOX C-ba base sed d mesh mesh restoratio restoration n architectures, architectures, depending on the service provider’s desired degree of bandwidth “overbuild” and survivability time-scale requirements. As similar bandw bandwidth idth-- manage management ment requir requireme ements nts emerge emerge for the metropolitan interoffice (IOF) and Access environments, environments, so so too will wil l OA OADM DM ring rin g-based -based

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solutions optimized for these applications: optical shared protection rings for mesh demands, and optical dedicated protection rings for hubbed demands. Hence, just as the optical amplifier (OA) was the technology enabler for the emergence of WDM point-to-point line systems, in turn, OADMs and OXCs will be the enablers for the emergence of the optical transport network. The optical layer will come to serve as the unifying transport infrastructure for both legacy and converged packet networks. Of course, service-provider movement to optical transport networking will be predicated on the transfer of the required transport-networking ncurrr ent wi th the functionality to the optical optical layer, layer, concur development of an OTN maintenance philosophy and associated network-maintenance features. This leads us to the key factors associated with realizing the vision of optical transport networking.

UILD DIN ING G A NEW TRANSPORT LAYER: BUIL ISIO ION N FO FOR R OPTICAL A PRACTICAL VIS  TRANSPORT NETWORKING

R

ealizing alizin g optical optical transport transport networking network ing architectures involves carefully balancing a web of tradeoffs in context with a set of critical factors, and successfully rising to meet associated technical and networking challenges. Implicit in the balancing exercise is the ubiquitous

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cost/functionality tradeoff – in other words, at what wh at price functionality? function ality? Rather Rather than a sing in gle solution for all applications, a range of optical-networking architectures will arise as each market segment applies its unique priorities to make fundamental architecture tradeoffs. In considering the key issues and cost/functionality trade-offs for next-generation optical transport networks, five critical factors emerge: • A nalog network network engin enginee eeri ring: ng:In In order to assure a high-performance network, it is necessary to limit the accumulation of analog impairments, impairments, while whi le balancing balancing cost cost with w ith other tradeoffs. A network-engineering strategy that enforces network integrity at subnetwork domain boundaries, resulting in a segmented approach to span engineering and regeneration, will help realize this goal • Service Transparency: In order to future-proof the network, and to simplify the engineering of the shared multi-service core, a strategy is needed to minimize the amount of  client-dependent processing within the core of  the optical transport network • Survivability: While Whi le continui continuing ng to offer offer the fastest possible recovery from faults, a strategy is needed to ensure that the next-generation transport network continues to offer a high degree of survivability coupled with network efficiency and service flexibility • Maintenance: A n ess essential element element to rea r eali lizing zing cost and service transparency goals is a main maintena tenance nce phi philosophy losophy tailored to the th e unique uni que needs and constraints of optical transport networking. networki ng. This Thi s OTN maintenance maintenance phi philosophy losophy should build on prior experience with TDM and FDM systems ystems,, incorporati i ncorporating ng useful useful featur features es,, while avoiding the mistakes of the past • Interoperability: As standards and underlying technologies mature, near- and mid-term evolutionary strategies are required to help ensure a smooth transition from single-vendor solutions to multi-vendor, interoperable optical transport networks

A NALOG NETWORK  ENGINEERING Despite Despite the th e apparent apparent similarity simil arity between between SONET/SDH and optical-networking architectures, the difference in design and implementation of 

these transport networks is fundamental. While the former is an exercise in digital network engineering, the latter is an exercise in analog network engineering, somewhat similar to the des design of freq f requency uency division division multiplexe multi plexed d (FDM (F DM)) networks of old. The chief advantage of digital transmission has been the ruggedness of the digital signal. The limiting factor in a properly designed digital system is not impairments introduced by the transmission facility, but rather the accuracy of the conversion of the original analog waveform into digital form. A major advantage has been that system noise is controlled by the design of the digital network element and is essentially independent of the total length of the system. Because no appreciable degradation is incurred in time division multiplexing or demultiplexing, facility arrangements do not typically need to take the number of previous multiplex/demultiplex operations into account. In contrast to TDM systems, the phase distortion and attenuation (and resulting noise degradation) suffered by channels at the band edge edge of FDM FDM systems systems can can affec aff ectt the th e system performance and so must be explicitly considered in engineering the total system [5]. In the excitement associated with advances in optical-networking technology, the implications of  the differences cited above have not always been sufficiently considered. In what is termed “all-optical” networking, signals traverse the network entirely in the optical domain, with no form of opto-electronic processing within wi thin opticaloptical-network network eleme elements nts.. This Th is implies impli es that all signal signal processing ing – includin including g signal regenera regeneration tion,, routing, routi ng, and and freq fr equenc uency y slot interc i nterchang hange e (FSI), (FSI ), or wavelength interchange –takes place entirely in the optical domain. Over recent years, there has been considerable debate over the nature of the term “optical” “optical” in optical optical networking networkin g; in i n retrospe retrospect, ct, it is clear that function and implementation were often confused. Given analog engineering constraints, and considering the current state-of-the-art in all-optical processing technology, the global, or even national, “all-optical” network is not attainable. In particular, opto-electronic conversion may be required in optical network elements to prevent the accumulation of transmission impairments – impairments that result from such factors as fiber chromatic dispersion and non-linearity’s, cascading of non-ideal flat-gain amplifiers, optical signal crosstalk, and transmission-spectrum-narrowing from cascaded non-flat filters [6]. Opto-electronic conversion can also support wavelength interchange, which is

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OTN Client Connection

Transparant Subnetworks, Sub networks, Bounded Boun ded by opto-electronics opto-elect ronics

Optical Engineering Subnetwork

OLS

OXC

OADM

Opto-electronic Processing Unit

Figure 6. Practical Optical Transport Networking Architectures currently a challenging feature to realize in the all-optical domain. In short, in the absence of commercially available devices that perform signal regeneration to mitigate impairment accumulation and support wavelength conversion conversion in i n the all-optica all- opticall domain, domain, some measure of opto-electronic conversion should be expected in practical optical-networking architectures. Because the use of opto-electronics is often associated with higher-cost solutions, the ensuing exercise in cost reduction involves establishing what constitutes the minimum required amount of opto-electronics necessary to assure the presence of desired optical transport networking attributes enabled by their presence. In the near term, the result of this exercise is reflected in optical-networking architectures characterized by transparent (or “all-optical”) segments, bounded by opto-electronics, as shown in Figure 6. Until standards for the OTN – which are in the early stages of development [7] – are fully developed, the near-term transparent subnetworks shown in Figure 6 will likely remain single-vendor solutions. Hence OXCs, as points of multi-vendor interoperability, are shown distinct from near-term, sing in gle-vendor OA O A DM and OLS-ba OLS- bas sed subnetworks. subnetworks.  The emerge rgence nce of OTN stand tanda ards rds will cha chang nge e this view, as described below.

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SERVICE TRANSPARENCY As we have seen in the preceding discussion, practical considerations will govern the ultimate realization of the Optical Transport Network (OTN). Paramount among these considerations is the network operator’s desire for a high degree of  service transparency within the future transport infrastructure. As the term “transparency” has tended to engender much discussion, we will attempt here to clearly define the intent of the term “service transparency”. SONET/SDH offers a good example of service transparency. Specifically, for a desired set of client signals targeted for transport on a SONET/SDH network, individual mappings are defined for carrying these signals as payloads of SONET/SDH server signals signals (along (alon g with ass associated ociated SONET/ SONE T/SDH SDH overhead to assure proper networking functionality). Once a client signal has been mapped into its SONET/SDH server signal at the ingress of the SONET/SDH network, an operator deploying such a network need not possess detailed knowledge of the client signal until it is de-mapped at the network egress. This definition of  service transparency is equally applicable to the OTN. Indeed, once a client signal is mapped into an optical channel, an OTN network operator should not require detailed knowledge of (nor access to) the client signal until it is de-mapped at the OTN egress. Hence, the optical-network ingress and egress points should delimit the domain of OTN servi service ce transparency. transparency.

LUCENT TECHNOLOGIES OPTICAL NETWORKING

Service transparency is an essential factor, because it helps control the complexity of the optical network’s role as a unifying infrastructure for all types of clients. In the context of optical transport networking, the notion of service transparency is currently limited to digital client signals, but the scope will ultimately expand to include analog client signals as well. Without service transparency, one must be aware of the type of client signal on every channel throughout the network, resulting in increased network engineering and maintenance complexity. The most important factor in realizing service transparency is to eliminate all clientclient-sp spec ecif ific ic equipment and processing processing between between OTN ingress and egress points. Fortunately, at the ingress/ ngress/ egress egress point, point, it is easier easier to accept accept client-dependent equipment, because it is generally dedicated dedicated on a per-se per-servi rvice ce basis anyw anyway. ay. For For example, we might expect different opto-electronic port units to be required to support a mix of 622 Mbps M bps,, 2.5 Gbps, Gbps, and 10 Gbps SDH/SONE SDH/ SONET; T; Gigabit Gigabit Ethernet; Eth ernet; emerg emergin ing g I P/WD P/ WDM M mappings; mappings; and leased “clear-channel” optical channels. Thus, service transpa transparency rency within with in the OTN minima mini mall lly y requires flexibility at the optical-channel layer to carry a wide variety of digital client signals. We will now examine the impact, and consequent implications, on service transparency of deploying optical-network architectures characterized by optically transparent segments bounded by opto-electronic processing. One immediate engineering consequence of such an architecture is that cascading opto-electronic processing units will introduce in troduce digital digital jitte ji tterr and wander wander accumulation accumulation within digital client signals carried on optical channels, unless they employ so-called  3R r egener gener atio ati on to regenerate, reshape, and retime these client signals. By virtue of this re-timing functionality, such devices have traditionally depended on the client-signal bit rate. However, bit-rate dependent opto-electronics result in constraining the service transparency of optical networks. The use of “broad-band” or bit-rate independent opto-electronic regenerators with re-timing functionality [8] will alleviate this constraint, thereby “opening up” the optical network to a wider variety of client signal bit rates. Such opto-electronic regenerators will enable cascaded OTN architectures composed of a balanced combination of optical transparency and “feature-enhanced” opto-electronics supporting bit-rate and protocol independence at the optical-channel layer. Bit-rate independent opto-electronic regenerators with retiming

functionality are therefore a key enabler for OTN service transparency.

NETWORK  M AINTENANCE  The evolutio volution n towa toward rd optic optica al tra transpo nsport rt netwo networking rking also brings further challenges to integrated telecommunications network management. A set of requirements must be established related to fault, configuration, and performance management, including the establishment of speed, latency, and robustness requirements associated with OTN management functions and communications. Indeed, when we examine OTN management needs, we can recognize many that are applicable to both SONET/SDH and OTN networks. However, it is essential to avoid the trap of assuming that whatever has been done for SONET/SDH is directly applicable to the optical transport network. Some of the forces that led to SONET/SDH standards were particular to that time in network evolution. For example, one of the main maintenance adva advantag ntages es for moving from PDH to SONET/ SONE T/SDH SDH was an opportunity to significantly expand, and more importantly to truly standardize, capabilities for monitoring the validity, integrity, and quality of  transport. transport. SONET/ SDH, with wi th its i ts digita digitall signal ignal format, offered a relatively painless means for these enhancements, by means of layered provision of  associated maintenance overhead. Network operators saw opportunities to attempt a proactive maintenance philosophy, fueled by a computationally intensive collection of masses of  data, and assign responsibilities for poor performance where a signal crosses networkoperator boundaries.  J ust ust as as SONET SONET/SDH /SDH offe offered red an oppo opportu rtunity nity to correct some of the operating problems of the PDH network, so too will the OTN provide an opportunity to correct some of the operating problems associated with the SONET/SDH network. For example: • A reasse reassess ssment ment of the types and and use u ses s of  performance information is in order. It has become clear that network availability has been largely unaffected by attempts at proactive maintenance strategies, and by the resultant abundance of SONET/SDH performancemonitoring information. Rather, network availability is primarily determined by components, design, and survivability decisions,

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so it seems reasonable to first focus energy on quick fault localization, to speed network up repairs. Volumes of detailed performance data at every node within a subnetwork will not contribute to fault localization. On the other hand, performance data has become absolutely esse essent ntiial at subnetwor subnetwork k iingress ngress// egress egress point points s for for verifying tariffs and troubleshooting between operators. It is fortunate that performance data will wi ll be readily readily available at thes these e ingress/ egress egress points, because they are very likely to have opto-electronic processing, and thus will offer relatively easy access to both digital and analog measurements • Two maintenance maintenance needs needs were not fully ful ly rea r eali lize zed d when the first SDH/SONET standards were put in place, and have proven difficult to realize and deploy in a widespread, practical fashion. OTN maintenance offers some hope that these needs may be better satisfied. The current difficulty of  knowing the physical topology of the network can be resolved by providing a trace function by means of the Optical Supervisory Channel (OSC) on each fiber segment. Additionally, requirements for tandem connection monitoring on signals that traverse multiple operator domains can be developed early in the standards process A gain, owing owi ng to the analog analog nature of WDM W DM,, establishing the performance of a client signal in an

OTN is a very different proposition from that for SONET/SDH systems. The difficulties related to performance monitoring within optical transport networks are inherent in the parameters that need to be measured. Specifically measured optical parameters must be accurately correlated with degradations in the client-signal layers in order to indicate that the optical layers are not providing the required level of service. It is becoming clear that establishing such a relationship using practical measurement techniques remains a significant challenge [9]. Aside from details of performance monitoring and measurements, a more general and fundamental challenge remains: what operations, admini adminis stration, and main maintena tenance nce (OA M ) information is needed at each point in the network, and how should it be carried? This information falls into two categories: information that must be associated with, and must follow, a particul particula ar optical optical channel channel connection connection;; and information that need not necessarily follow this connection. It is the former that will provide the main challenge, because the latter can be carried in the optical supervisory channel, or through some other external means. means. Our practical optical-network vision calls for a maintenance strategy aligned with an engineering approach of transparent optical subnetworks bounded by opto-electronics. As illustrated in

Thorough Performance Info at Subnet Boundaries (especially for tariff/SLA verification)

OTN Client Connection

Intra-subnet Maintenance Targets Fault Location

Optical Engineering Subnetwork

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OADM

Fig ure 7. OTN M aintenanc aintenance e

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Opto-electronic Processing Unit

Figure 7, the maintenance strategy, out of  necessity, becomes a balanced combination of  “opto-electronic enabled maintenance” (where opto-electronics are present) and “off-line,” shared, non-intrusive measurement and monitoring of the optica opticall llyy-transpa transparent rent segm segments. ents.

NETWORK  SURVIVABILITY Survivability is central to optical networking’s role as the unifying transport infrastructure. As with many other architectural aspects, optical-network survivability will bear a high-level resemblance to SONET/SDH survivability, because the network topologies and types of network elements are very similar. Within the optical layer itself, survivability mechanisms will continue to offer the fastest possible recovery from fiber cuts and other physical-media faults, as well as more efficient and flexible management of protection capacity. Recovery times will range from below 50 milliseconds to a few hundred milliseconds, depending on the particular network architecture and the type of fault. There are a variety of  survivability schemes that may be considered: For point-to-point WDM line systems involving WDM -TM s and and WDM -OA s, 1+1 optical optical protect protection ion switching schemes similar to those for SDH/SONET are often considere considered. d. The addition addition of the OA DM to the WDM line li ne sys system tem will allow more adva advance nced, d, optical-layer protection switching schemes, again analogous to those for SDH/SONET. Simple architectures architectures for optical opti cal add/ add/drop, drop, such as 1+1 Optical SubNetwork Connection Protection (O-SNCP), are single-ended and thus do not require coordinated Automa Au tomatic tic Protec Pr otection tion Switching (APS) algorithms, protocols, and signaling channels. They can be applied to both linear add/drop and ring topologies. M ore complex, complex, opticaloptical-laye layerr protection protection switching wi tching architectures will be capable of greater flexibility and bandwidth efficiency, especially for meshed traffic demand patterns. For ring topologies in core networks, an optical-layer version of the SDH/SONET Shared Protection Ring is a likely candidate.  The introd introduc uctio tion n of OXCs will ena enab ble optic optica al-laye l-layer mesh-based architectures. For some applications, a mesh architecture will be lowest cost, largely due to intelligent, automated mesh restoration schemes that can be highly optimized for efficient allocation of protec pr otectio tion n capacity. capacity. M eshesh-ba base sed d restorati restoration on

algorithms algorith ms have have been been develo develope ped d for SONET/ SONET/ SDH networks; however, however, the th e introduction of new n ew optical optical technology and associated network elements offers opportunities for enhancements in restoration spee speed d to support survivab survi vable le optical optical trans tr ansport port network architectures. For example, distributed mesh-based restoration schemes can be employed that not only increase restoration speed, but also accommodate churn due to frequent provisioning and rearrangements, with associated algorithms that can interact constructively with a centralized capacity planning system. While the general classes of survivable optical-network architectures are similar to those in SDH/SONET, there are some fundamental differences in functionality and applications. Survivable optical optical networks must provide flexible support for the requirements of a wide range of  client signals, from embedded SONET/SDH rings that simply require unprotected access to WDM wavelengths to highly optimized optical-layer restoratio restoration n scheme schemes s capab capable le of of differentiatin diff erentiating g among the needs of different service classes in a converged packet core. Additionally, survivable optical networks must be even more bandwidth efficient in order to support the tremendous tremendous bandwi bandwidth dth carri carried ed on WDM links. li nks. These These gains gains in client networking flexibility and bandwidth efficiency also rely on a coherent multi-layer survivability strategy in which optical layer survivability mechanisms coexist gracefully with those of its client signals (for example, SONET/SDH, SONET/SDH, ATM , and IP). Issues associated with multi-layer survivability have been cited within recent conferences (see [10]) and research consortia (see [11]). For instance, in the SONET world, consider an ATM OC-3/OC-12 based backbone network over an SONET OC-48 ring, with some of the SONET spans carried as optical channels over an OTN system. Each Each of these these three layers layers – ATM A TM,, SONET, SONET, and WDM WDM – may have survivabil urvivability ity mechani mechanis sms; ms; however, the absence of a cohesive multi-layer survivability approach may cause contention among the layers and result in inefficient operations and unnecessary survivability actions. M ore gene generi rica call lly, y, there are are two pitfalls pitf alls to avoid avoid in in multi-layer survivability, as illustrated in Figure 8.  The first first invo involve lves s the pote otential ntial for for cons conside idera rab ble wasted bandwidth bandwi dth, because each layer reserves its own backup resources to use during failures. The second pitfall relates to the fact that a single physical-layer failure (such as a fiber cut) can

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P itfall :

P itfal itfalll :

Was te ted d B andwidt ndwidth h

Unneces Unne ces s ary P rote rotection ction A ctions

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ON

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Fig ure 8. Common Multi -Layer -Layer Survivability Survivability Pitf alls alls

un nece essary ssar y prote pr otection ti on acti actions ons at the result in many unnec client layers. This undesirable result can greatly extend the end user’s measured outage from a single fault, potentially activating the “minimum availability” penalties of service-level agreements, as well as complicating the carrier’s maintenance tasks. Considerable discussion is taking place within the indus in dustry try regard regardin ing g poss possible multimul ti-laye layerr surviva urvi vabili bility ty strategies. However, given the range of operator policies, services, and deployed technologies, it is clear that any proposed approaches must enable flexibility that allows them to be efficiently and cost-effectively tailored to individual operator needs. For example, an ideal approach could involve in volve deployin deploying g only one survi survivab vabil ility ity scheme cheme

per subnetwork domain per service, coupled with availability of provisionable hold-off timers for optional premium service and restoration support. As a simple case, we can consider that for most services, an optical-layer protection or restoration scheme will be desirable, because it can offer a unifying solution. Sometimes, though, it may be best to use the client layer’s survivability scheme, and disable optical-layer protection on that client’s optical channel. In summary, optical network survivability will continue to offer the fastest possible recovery from faults, while catering to a wide diversity of client signals having varying survivability requirements.

OTN Client

OTN

Connection

Client Connection

Interconnection Among Idependent "OTN Islands" via Single-Channe Single-Channell Client or WDM Physical Layer L ayer Connection Optical Engineerin Engineering g Subnetwork

OLS

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Fig ure 9. I nterconnec nterconnecting ting OTN Islands Islands

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Opto-electronic Processing Unit

OCH Endpoint (OCH Continuity Maintaind Across the Network)

OTN Client OTN

Connection

Client Connection

OTN-Level Interworking : No longer Interconnecting OTN "Islands" via OTN-Client Level Signals OCH-Level Continuity is Maintained Across the Optical Transport Network Transparent Subnetwork

OLS

OXC

OADM

Opto-electronic Processing Unit

Fig ure 10. OTN-Lev OTN-Level el Interwor ki ng

I NTEROPERABILITY  The applic pplica ation tion of any ne new tec technolog hnology, y, such uch as as WDM , in i n the th e telec telecomm ommuni unica cations tions environment requires that standards be developed to facilitate multi-vendor interworking and network interconnection. A key aspect is to fully define the information that is associated with the Optical Network Node Interface (NNI) – for example, the format of the optical supervisory channels that carry OAM information between network elements. Another key aspect involves the physical layer and requires a very precise description of the physical properties of the multi-wavelength optical signal. Not only is this an extremely difficult task to achieve for what is essentially an analog network-design problem, it is hampered by the rich diversity of options for optical transmission [7]. For example, the optimal choice of operating wavelengths wavelengths for a WDM line li ne system system is depe dependent ndent on a number of factors, including: type of fiber, OA technology, filter technology, span distance, and overall target reach for the system. In general, the choice of operating wavelengths is dictated by a combination combination of underlyin u nderlying g technology technology choices choices coupled with the envisioned application(s). Given the range of specifications that need to be stabilized, and the pace of technology advances, interoperability is expected to follow an evolutionary path. As a first step, multi-vendor interworking will likely be achieved by way of  single-channel OTN, client-level interconnection (for example, SONET/SDH interfaces conforming to existing standards, such as ITU-T Rec. G.957). This

is shown in Figure 9; again, note that the OXCs, as points of multimul ti-ve vendor ndor interope i nteroperab rabil ility, ity, are distinct distinct from the single-vendor OADM and OLS-based subnetworks. In the near future, multi-channel OTN, client-level interconnection will occur by using using standa standard rd WDM W DM wavelengths and physical-layer physical-layer parameter parameter specifi specifica catio tions. ns. The The above solutions imply interconnection among a network of “OTN islands” [12]. In the future, single-channel or multi-channel, OTN-level interworking will occur when full OTN-NNI OTN- NNI specifica pecifications tions are availab available, le, includin i ncluding g the specification of optical-layer overheads and the Optical Supervisory Channel. As OTN standards mature, the level of interworking will allow for optical optical channelchannel-leve levell continuity continui ty and optical optical networking beyond constrained transparent segments. As shown in Figure 10, OTN-level interworking will permit the independent “OTN islands” to grow, and in some cases grow together, creating larger administrative and span engineering subnetworks.

REAL EALIZ IZING ING THE OPTICAL NETWORKING VISION

 T

he transfer of transport-layer functionality to the Optical Transport Network, concurrent with the development of a maintenance philosophy and ass associated ociated OAM OA M featur features es,, will wi ll enable enable evol evoluti ution on from single-channel optical-transmission systems to OTNs with advanced features such as optical

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channel routing and wavelength conversion/interchange. It will also facilitate realizing the network operator goal of a flexible, scalable, and robust transport network, catering to an expanding variety of client signals having equally varied service requirements (flexibility, scalability, and survivability coupled with bit-rate and protocol independence). The evolutionary path to the ultimate goal of a unified transport layer will involve a number of key considerations, including the optimization of optical-layer transport functionality while maintaining clientindependence. The optical layer, to remain futureproof, ins in sofar as future-proofing future-proofi ng is poss possible, should not be optimized for any particular client signal (this (thi s includes includes the opticaloptical-laye layerr control plane). plane). J ust as the transport network has evolved to new signal formats, so too will today’s “networks du jour.” We need to avoid “locking-in” the optical layer to a myopic view of the future, based solely on legacy  TDM  TDM and leg legacy (“b (“best-effo t-effort” rt”)) da data signa ignals ls.. It should be emphasized that SONET/SDH will continue continue to be an an integ i ntegral ral part of the data/opti data/optica call “convergence” networking evolution. While transport-networking responsibility will shift (dramatically, perhaps) over time, from the SONET/SDH layer to the optical layer, SONET/SDH will continue to serve the networking role for modest aggregate point-to-point bandwidth demands, such as voice trunking and many traffic types at the edges of the network. Some new telecommunications carriers will aggressively build packet-based networks from the ground up, and therefore will not have to deal with integration of  legac legacy y voice/TDM networks. On the other hand, the vast majority of carriers will find a way of  integrating their current networks with emergent network infrastructures based on the optical layer. Building the high-capacity optical transport network of the future depends on innovative solutions incorporating cutting-edge technologies coupled with sound networking principles. abs Tec Techni cal Jo J our nal on A Special Issue of the B ell L abs Optical Networking outlines the Lucent Network Vision for leading the transition to a network of  networks unified by Optical Networking, highlig highli ghting htin g Lucent’s Lucent’s innova inn ovative tive soluti solutions ons for:

converged optical data network [16][17] • Optical Optical Networkin Network ing g – the core core of the next generation eneration ne n etwork infras inf rastructure tructure [18], from research to realization [19] • M igrating igrating Optica Optical Networki Networking ng toward toward viable viable metropolitan interoffice and broadband business-access transport solutions [20] • Critica Cri ticall components components for optical optical networking – optical amplifiers [21], optical add/drop technologies [22], and the future of  high-capacity transport [23] • Building Buil ding blocks blocks of the Optica Optical Networking Networking infrastructure, from fiber [24] to nextgeneratio eneration n optical cross- connects [25] • Optical Optical Networking Network ing s soluti olutions ons for enterprise enterprise and residential access applications [26][27]

SUMMARY

A

revolution in data networking fueled by the

explosive growth of the Internet is in volutitio on of  significant measure responsible for the evolu transport networking toward an Optical Networking infrastructure. Optical Transport Networking will leverage the transport infrastructure in the era of data/transport convergence convergence by offeri of fering ng carriers carriers unprec unprecede edented nted architectural flexibility – client-protocol (and bit-rate) independence, and service differentiation by separation of optical channels for different types of servi service ces s (TDM (TDM , ATM ATM,, IP, I P, optical leas leased lines li nes,, and so forth). With a practical vision for Optical  Trans  Transp port Netwo Networking rking,, a balanc lance ed conside onsidera ratio tion n of  analog network engineering, service transparency, surviva urvi vabili bility, ty, maintenance maintenance,, and inte in terope roperab rabil ility ity will wi ll render a cost-effective, survivable, and flexible broadba broadband nd opticaloptical-trans transport port infras in frastructure. tructure. I n short, we do not have long to wait for an opticaltransport network that can rise to the challenge of  providing an optimized layer of high-capacity, high-reliabili high- reliability ty bandwi bandwidth dth manag managem ement ent that includes multi-service support is close at hand.

• The evolution evolution of fle fl exible ba bandwidth management [13][14] and survivable network design [15] in the era of converged data data// transport transport networking networki ng • A rchitecture rchitecture and and protocol considera considerations tions for a

18

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A CKNOWLEDGMENTS

 T

REFERENCES

he authors would like to thank the many friends and colleagues in the Lucent community who have (in one way or another) contributed to the material presented in this paper. Specia Speciall thanks are are extende extended d to J on A nderson, nderson, J ohn Eaves, and George Newsome for their insightful comments during the preparation of this manuscript.

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GLOSSARY Abbreviation bbreviations s Used: Used: ADM

A dd/ Drop M ultiplexer

APS

A utomatic Protection Switching

ATM

A snychronous Transfer Transfer M ode

DCS

Digital Cross- Connect System

DWDM

Dense Dense Wavelengt Wavelength h Divis Divi sion M ultiplexin ul tiplexing g

EDFA

Erbium Doped Fiber A mplifier

FR

Frame Relay

I OF

I nteroffice

IP

I nternet Protocol

ITU

I nt nternational Telecommunication Union

kbps

kilobits per second

M bps

M egabits per second

NNI

Network Node I nterface

O-SNC -SNCP P

Opt Optica ical Sub SubNe Nettwork work Conn Conne ection ion Pro Prottection ion

OA

Optical A mplifier

OA DM DM

Optical Ad Add/ Dr Drop M ul ultiplexer

OAM OA M

Operations, Operations, A dministration, dministration, and M aintenance aintenance

OCh

Optical Optical Channel

OL S

Optical Line System

OSC

Optical Supervisory Channel

OTN

Optical Transport Network

OXC

Optical Cross Connect

PDH

Plesiochronous Digital Hi Hierarchy

PSTN

Public Switched Telephone Network

SDH

Synchronous Digital Hierarchy

SNCP

SubNetwork Connection Protection

SONET

Synchronous Optical Network

 TDM

Time Time-Division -Division Multiple Multiplexing xing

 TM

Term Termina inall Multip M ultiple lexe xerr

 Tbp  Tbps

Tera Terab bits per se second

VPN

Virtual Vir tual Private Network

WDM

Wavelength Di Division M ul ultiplexing

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 This  This docume ument is for for pla planni nning ng purpos urposes only, and is not intended to modify or supplement any Lucent Technologies specifications or warranties relating to these products or services. Performance figures and data quoted in this document are typical and must be specifically confirmed in writing by Lucent  Techno hnolog logies ies before fore they hey become ome applicable to any particular order or contract. The company reserves the right to make alterations or amendments to the detailed specifications at its discretion.  The  The public ublica ation tion of infor inform mation tion in this this document does not imply freedom from patent or other protective rights of Lucent Technologies or others. WaveStar and AllWave are trademarks of  Lucent Technologies Inc.  To lea learn more ore about out our our comp omprehe rehens nsive ive portfolio and the new WaveStar™ ONG Series, please contact your Lucent  Technolo hnolog gies ies Sales les Rep Repres resenta ntative tive or call 1-888-4 LUCENT. Visit our web site at http://www.lucent-optical.com Copyright © 1999 Lucent Technologies Inc. All rights reserved Printed in Holland / USA Lucent Technologies Inc. M arketing arketing Communicat Communications ions Order Number: WP-004/990502 8WP Issue 2 M CO/BA CO/BAP P 05 05/99 /99

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